Pg 1030-1067 Chap4-OptimisedCementDesign Text

_<;MI.H:i:MfTTgl"Holderbank" Cement Seminar 2000

Materials Technology 111 - Optimised Cement Design

Chapter 4

Optimised Cement Design

© Holderbank Management & Consulting. 2000 Page 1 01

"Holderbank" Cement Seminar 2000

Materials Technology III - Optimised Cement Design

Page 102 © Holderbank Management & Consulting, 2000



OPTIMISED CEMENT DESIGN

1 . INTRODUCTION 1 05

2. BASIC CONSIDERATIONS for cement design 105

2.1 Product requirements 105

2.2 Available cement components 106

2.3 Production facilities 107

2.4 Economy 107

3. Influence OF CEMENT COMPONENTS on CEMENT PROPERTIES 107

3.1 General 107

3.2 Clinker 108

3.2.1 Water requirement of standard paste and consistency of concrete 109

1 .1 .2 Rate of stiffening and setting time of standard paste and slump loss of concrete 1 1

1.1.3 Heat of hydration 110

1.1.4 Strength of mortar and concrete 110

1.1.5 Sulphate resistance 115

1.1.6 Other properties 116

1.2 Mineral components 116

1.3 Gypsum 117

1.4 Chemical admixtures 118

2. OPtimum PROPORTIONING of the cement components 118

2.1 General 118

2.2 Clinker and mineral components 118

2.3 Gypsum 119

2.4 Chemical admixtures 120

3. Cement grinding 120

3.1 General 120

3.2 Description of fineness 120

3.2.1 Specific surface area 120

3.2.2 Particle size distribution 121

3.3 Particle size distribution in the different grinding systems 123

3.4 Grinding of Portland cements 123

3.4.1 Influence of Blaine fineness 123

3.4.2 Influence of particle size distribution 127

© Holderbank Management & Consulting, 2000 Page 103

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'M . ] d I • rV I 'EM'Holderbank" Cement Seminar 2000 — i-^^gmMaterials Technology III - Optimised Cement Design

3.5 Grinding of blended cements 129

3.5.1 General 129

3.5.2 Compound grinding 130

3.5.3 Separate grinding 130

3.6 Temperature and moisture conditions 134

4. LITERATURE 138

Page 1 04 © Holderbank Management & Consulting, 2000

'Holderbank" Cement Seminar 2000—i:i-3vn;j:f:iy^


1. INTRODUCTION

The process of cement design consists of the following interrelated steps:

1

)

selection of the most convenient set of components

2) determination of the relative proportions of the components

3) definition of the fineness and grain size distribution of cement (compound grinding) or

the cement components (separate grinding)

The objective of the cement design is to achieve the specified or desired performance of the

cement at the minimum possible cost.

The procedures applied in practice for cement design are usually based on analytical tools

(models to predict cement performance), experiments (trials on laboratory and industrial

scale) and on experience. The better the knowledge of the relationships between cementcomponents, proportioning and processing and the cement properties, the easier it is to

arrive at the optimum solution.

The optimisation of the cement design requires nowadays more attention than in earlier

times. The main reasons for this development are:

increase in number of cement components (use of mineral components and chemical

admixtures in the cement)

use of new grinding technologies (roller press, vertical mill, Horomill) having an effect onthe resulting grain size distribution and grinding temperature

extension of product and application range

The object of the present paper is to describe the basic considerations influencing the

cement design and to give an overview on the influence of the cement components, their

proportioning and cement grinding on the properties of cement.

2. BASIC CONSIDERATIONS FOR CEMENT DESIGN

2.1 Product requirements

The cement design will strongly depend on the performance requirements to be fulfilled bythe cement. These requirements, which are determined by the respective standards and bythe market, may comprise specifications on.

proportioning of cement components and chemical composition

workability (water demand, setting), volume stability and strength

special properties:

• heat of hydration

• sulphate resistance

• alkali-aggregate reactivity

• shrinkage, etc.

Besides the above specifications, there may be further requirements with regard to the

handling of the cement (i.e. temperature, flowability and storage stability).

In the future, also certain requirements with respect to energy consumption and emissions

(in particular C02) during the cement production may be imposed.

© Holderbank Management & Consulting, 2000 Page 1 05

"Holderbank" Cement Seminar 2000Materials Technology III - Optimised Cement Design

!MJ.»:J:MJIT

2.2 Available cement components

The flexibility in cement design will be obviously controlled to a large extent by the available

cement components. The most important aspects of the cement components (clinker,

mineral components and gypsum) in this respect are:

available quantities

quality / uniformity and

costs

The cement plants usually count with one "normal" type of clinker, whose characteristics are

pre-determined by the raw material situation and by the burning and cooling conditions in

the kiln. Occasionally, also special clinkers are produced for certain cement types, but with

the increased use of mineral components in the cement, which allow to obtain special

properties with "normal" clinker, less and less of such clinkers will be applied in the future.

The availability of the mineral components varies from country to country. The mainindustrial by-products used for cement production - blast furnace slag and fly ashes - are

principally available world-wide in great quantities (see Table 1); however, only part of it

complies with the necessary quality requirements for an application in the cement. Amongthe natural mineral components, limestone of suitable quality should be available at all

cement plants, whereas the natural pozzolans are less wide-spread.

Table 1 : Estimated production of fly ashes and blast furnace slags (Mio t/a)

Blast furnace slag (1994) Fly ash (1992)

Western Europe 36 61

Eastern Europe (+ former

USSR)28 95

North America 20 51

Latin America 11 3

Africa 3 24

Asia 78 125

Australia 3 7

World 178 366

Natural gypsum deposits are scarce in some countries. In such cases, alternative materials

like natural anhydrite and limestone or by-product gypsum from other industries have to beconsidered.

The different type of chemical admixtures, which can be added at the cement grinding

stage, can in principle be purchased anywhere in the world.


"Holderbank" Cement Seminar 2000—i:LMi-]=M=f:i^f


2.3 Production facilities

The available production facilities (in particular the cement grinding installations) put certain

constraints on the cement design. Such constraints may lead to limitations with respect to:

clinker factor

fineness range of cements

feasible number of products

type of cement grinding (compound or separate)

Moreover, the cement design will be influenced decisively by the type of cement mill used

for grinding. For instance, with the new grinding technologies, adjustments have to be madeto account for the differences in grain size distribution and grinding temperature comparedto the traditional systems.

2.4 Economy

The production costs of cement basically consist of the costs of the materials entering the

cement mill and the grinding costs. From the two factors, the material costs generate by far

the greatest part of the production costs.

The most expensive material in the cement is usually the clinker. The minimisation of the

clinker content in the cement is therefore the single most important factor in reducing the

production cost, provided that mineral components of suitable quality are available at

convenient prices.

The principal ways to reduce the clinker factor in the cement are:

adjustment of the fineness and grain size distribution of the cement and its components

use of high quality clinker

use of chemical admixtures already in the cement

The possible clinker reduction is of course limited by the factors discussed in the previous

chapters.

3. INFLUENCE OF CEMENT COMPONENTS ON CEMENT PROPERTIES

3.1 General

Due to the great variety of factors involved, it is difficult to describe precisely the relationship

between the cement components and the cement properties. The available models for the

prediction of cement performance usually only reflect the general trends.

The effects on the cement properties are best understood for the clinker and gypsum. Least

knowledge is available in case of the mineral components and the chemical admixtures, so

that virtually the only way to assess their influence is to carry out performance tests.



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3.2 Clinker

The composition of clinker gives some indications on the properties of cement to be

expected, as it influences the rate of hydration reaction and thus the setting and hardening

rate of cement The composition of clinkers control the quantity and rate of heat evolved

during hydration and the resistance of cement to sulphate attack; therefore, limiting values

are specified.

In this section, the influence of composition of clinker on the following properties of cement

shall be discussed:

water requirement of standard paste and consistency of concrete

stiffening rate and setting time of standard paste and slump loss of concrete

heat of hydration

strength of mortar and concrete

sulphate resistance

other properties of concrete

A summary on the relationship between clinker composition and the principal cementproperties workability (water demand, setting) and strength is given in Table 2.

Table 2: Effect of clinker composition on water requirement and setting time of

standard paste and compressive strength of ISO mortar (general trends)

Clinker Water req. Setting time Strength

early final

C3S ~ ~ 71 71

C2S — — N 71

C3A 71 ^ 71 NJ

C4AF — — NJ 71

K20 71 — 71 bJ

Na20 71 — 71 iJ

S03 — 71 71 id

P205 — 71 id —

7\ increasing

ii decreasing

- no effect

Page 108 > Holderbank Management & Consulting, 2000



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3.2.1 Water requirement of standard paste and consistency of concrete

The water requirement of the standard paste of normal consistency depends primarily on

the aluminate and alkali content of clinker and on the fineness of cement. From a multiple

regression analysis carried out at HMC on 48 different ordinary Portland cements, the

following relation between the water requirement and cement composition was derived:

W.r. % = 17.4 + 0.15 a + 0.26 b + 0.12 c

a = particle size fraction 1 to 30 141 in wt %b

c

C3A content in wt % (Bogue's formula)

total alkali content in wt %The relation between the water requirement of standard paste and the composition of

cement cannot be applied to concrete, as there is a rather weak relationship between the

water requirement of paste and water/cement ratio of concrete (see Figure 1

)

Figure 1 : Water requirement of cement and w/c-ratio of concrete

Concrete consistency

7.5 cm slump Qm DuHoi

Ho 2


'Holderbank" Cement Seminar 2000'


Po - Water reducing admixture (Pozzolith)

Me - Superplasticizer (Melment)

Lu 2 ... Ho 2 - various OPCGm, Du, etc. - various Group plants

The effect of cement on the consistency or water requirement of concrete is rather small

compared to other factors, such as sand, admixtures and temperature. An exception is

concrete with a very short mixing time, where cement with false set may seriously impair the

consistency of concrete.

3.2.2 Rate of stiffening and setting time of standard paste and slump loss of concrete

The stiffening rate or the „Vicat" setting time of the standard paste is significantly influenced

by the composition of clinker. The sulphates and phosphates of clinker usually delay,

whereas aluminate shorten the setting time of cement.

The relation between the stiffening rate or setting time of standard paste and the stiffening

rate - expressed as slump loss - of concrete is, just as for the water requirement, rather

poor. Therefore, it is difficult to estimate the stiffening rate of concrete on the basis of

composition or fineness of cement.

3.2.3 Heat of hydration

The effect of the clinker composition on heat of hydration has already been discussed in

detail in the paper on cement hydration. The principal way to control the heat evolution of

the clinker is the adjustment of the C3S and C3A content.

3.2.4 Strength of mortar and concrete

The rate of strength development of mortar or concrete depends on the type (or

composition) of cement. The general tendency of cements with a slow rate of hardening is

to have a slightly higher ultimate strength.

The ASTM type IV cement, with low content of C3S, has the lowest early strength, but

develops the highest ultimate strength (see Figure 2). This agrees with the influence of

individual clinker components on the rate of strength development measured on pure clinker

minerals (see Figure 3).



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Figure 2: Strength development of concrete made with different cement types

CM

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20

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II 1 I1

I

V.3 7 28 90

v ^days

i 5

yearsFigure 3: Compressive strength of cement compounds

360 days


1

•'Holderbank" Cement Seminar 2000


The two calcium silicates develop the highest strength, but at different rates. The aluminate

develops little strength, despite a high rate of hydration.

The rate of strength development of mortar or concrete depends on the clinker composition

as follows:

a) Calcium silicates. The different rates of hydration of C3S and C2S affect the rate of

hardening in a significant manner. A convenient rough rule assumes that C3Scontributes the most to the strength development during the first four weeks and C2Safterwards. In general, somewhat higher ultimate strengths are reached by cements with

lower calcium content, i.e. rich in C2S. This observation corresponds with the

assumption that the strength of cement depends on the specific surface of its hydration

products. C2S produces more colloidal CSH gel and less of the crystalline Ca(OH) 2 than

the C3S.

b) Aluminates and ferrites. The influence of the other two major components on the

strength development is still controversial. Presumably, the C3A contributes to the

strength of the cement paste during a period of one to three days, in general, both

aluminate and ferrite contribute to the strength of cement to a minor extent, but

significantly influence the hydration process of the silicates and thus have an indirect

effect on the rate of hardening.

c) Of the minor components, the alkali sulphates exert the greatest influence on the rate

of hardening. The alkali sulphate - mostly present as easily soluble potassium sulphate

or calcium-potassium sulphate with a molar ratio of 2:1 to 1:2 - accelerates the rate of

hardening, improving the early strength and decreasing the 28 day and ultimate strength

(see Figure 4). Of the other minor components, fluorine accelerates, whereas the

phosphorous compound delays the rate of hardening.

d) Clinker characteristics other than chemical composition. Particularly the burning

and cooling conditions influence the rate of hardening of a particular clinker composition.

Frequently, clinkers of the same chemical composition have different strengths and

clinkers of different chemical composition have the same strength. A simple experiment

proves that the very same clinker composition may have rates of hardening which vary

considerably. Rebuming of a clinker in a laboratory furnace changes the rate of

hardening, but does not affect the chemical composition of clinker (see Figure 5).



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Figure 4: Effect of soluble K20 on the compressive strength of ISO mortar

28 DAYS

r « 0,597

2 DAYS

r - 0.805



'HOLDERBANK'

Figure 5: Model of strength development of mortar and concrete made with twoclinkers of same chemical composition and different activity

L.

2 days;

K

28 daysi

MATURITY+

MORTAR

CONCRETE

®A

B

Maturity =

Clinker of high activity

Clinker of low activity

Degree of hydration

®A general guide on the necessary amount of the main clinker phases to achieve optimumstrength development is given in Table 3. The most essential point is to have a high C3Scontent (in the order of 60%) and to adjust the C3A content.



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Table 3: "Ideal" composition of the clinker for optimum strength development

Clinker phase "Ideal" content (%)

C3S

C2S

C3A

C4AF

55-65

15-25

7-10

7-10

The influence of the clinker composition on the standard mortar strength is noticeably

reduced in concrete. Depending on the quality of cement, sand and aggregate, the

proportioning of concrete or mortar components, curing temperature, specimen dimension,the rate of hardening in mortar and in various concrete compositions is quite different.

Moreover, the use of admixtures in concrete - which is common practice today - makes therelation even more complicated. Due to different hardening rates of mortar and concrete, therelation between concrete and mortar strength at various ages varies and depends on theabove mentioned factors. Concluding, the cement properties, as demonstrated throughstandardised testing methods, do not show their effect in the same way in concrete.

3.2.5 Sulphate resistance

The sulphate resistance of concrete depends primarily on the C3A content of clinker. Theferrite phases (C4AF) affect the sulphate resistance to a much lesser degree. The higherthe C3A content of clinker, the more susceptible the concrete is to sulphate corrosion (seeFigure 6).

Figure 6: Sulphate resistance of cement measured on ISO mortar specimen (55OPC)Influence of C3A content on the loss of Young's modulus of elasticity E(determined from ultrasonic pulse velocity measurements)

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5

"Holderbank" Cement Seminar 2000—i.i n m.i-t.\.i,<w


E28 after 28 days of regular curing = 1 00%

E1 80 after 28 days of regular curing and 1 80 days of

exposure to 10% sodium sulphate solution

The rate of sulphate corrosion depends - apart from the C3A content of clinker - on factors

other than cement:

composition of concrete, particularly the water/cement ratio

age of concrete at the time of the first exposure to sulphates

type and concentration of sulphate solution

duration and mode of sulphate exposure

3.2.6 Other properties

The other properties of concrete, such as

freeze - thaw - resistance

permeability

cracking

shrinkage and creep

are only slightly influenced by the composition of clinker and quality of cement. Otherinfluencing factors, such as air content, w/c-ratio, curing conditions, are decisive.

The cement exerts only an indirect influence on these properties by its effect on the waterrequirement and rate of hardening.

3.3 Mineral components

(see also paper on blended cements)

The effect of the mineral components on cement performance can be related mainly to their

activity. The three main classes of materials in this respect are latent hydraulic (e.g. blast

furnace slag), pozzolanic (e.g. fly ash and natural pozzolans) and inert (e.g. limestone).

In case of the active mineral components (latent hydraulic and pozzolanic), the generaleffects with respect to cement properties are as follows:

lower water requirement (except for natural pozzolans)

delay in setting times

lower heat of hydration

lower early strength

higher long term strength

lower permeability

improved resistance to sulphate and other chemical attacks

lower sensitivity for alkali-aggregate reaction



!Mi.»:i;?.-i*r?aa

The actual influence on the cement properties will of course still depend on the individualnature of each material. A more detailed comparison on the effects of the main activemineral components blast furnace slag, fly ash and natural pozzolan (at same dosage) is

made in Table 4.

Table 4: Effect of main active mineral components on cement properties (generaltrends)

Blast furnace slag Fly ash (class F) Natural pozzolan

Water requirement ij 71

Setting time 71 7171 7171

Heat of hydration ^1 ilSJ ^1^1

Early strength ^ ^iJ ^JNJ

Final strength 71 71 71

Sulphate resistance 71 7171 7171

Permeability (chloride) ^J M^J iJiJ

Alkali-aggregate iJ ^NJ iJil

reactivity

Shrinkage 71

7i increase

iJ decreaseO neutral effect

The inert mineral components like limestone do exert similar influences as the activematerials in terms of water requirement, setting and heat of hydration, but they will notimprove the final strength and durability characteristics of the cement.

Other cement properties than the above mentioned are generally not affected to a greatextent by the addition of mineral components.

3.4 Gypsum

(see also paper on cement hydration)

The main function of the gypsum in cement is to regulate the cement setting, but thegypsum also influences other cement properties such as grindability, flowability and storagestability, volume stability and strength.

The use of anhydrite instead of gypsum helps to reduce the risk of false setting and toimprove the storage stability and flowability of the cement at high grinding temperature (seealso chapter 5.6). In case of highly reactive clinkers, proper set retardation may, however,be a problem and blends with gypsum have to be used.

The substitution of natural gypsum by by-product gypsum may sometimes cause problemswith setting and strength development due to potential presence of impurities in such type ofmaterials.


Holderbank Cement Seminar 2000Materials Technology 111 - Optimised Cement Design

3.5 Chemical admixtures

The chemical admixtures, which can be added at the cement mill, are divided into the two

following main groups:

grinding aids having mainly a positive effect on the grinding energy

performance modifiers influencing significantly the cement quality, in particular water

requirement and strength development

The first group of admixtures (typically organic compounds based on alcohol and amines)

do as mentioned not really change the engineering properties of cement. The action of the

grinding aids is based on the reduction of the adhesive forces between the cement particles.

They may, however, facilitate the handling of the cement due to the resulting improvementin flowability.

The performance modifiers are of similar nature as the products used in the concrete mix.

Such admixtures are generally based on accelerators and water reducers and thus improve

the workability and strength development of the cement.

Needless to say that the use of chemical admixtures is only worthwile if there is a real

benefit with regard to the economy or performance of the cement to be produced.

4. OPTIMUM PROPORTIONING OF THE CEMENT COMPONENTS

4.1 General

The proportioning will be discussed here mainly from the point of view of cementperformance. The economic aspects, which are of course of primary importance for the

proportioning (see chapter 2.5), will not be dealt with, as they greatly depend on the specific

circumstances.

For a given cement performance, the proportioning is basically controlled by the quality of

the available cement components and the selected fineness of the cement and its

components. Further limitations are set by the standards, which specify the permitted

contents for the different cement components.

4.2 Clinker and mineral components

Portland cements

In case of the Portland cements, the flexibility in proportioning of clinker and mineral

components is obviously limited. According to the European Norm, up to 5% of mineral

component can be added to the cement, whereas ASTM does virtually not allow the addition

of mineral components besides clinker and gypsum. The focus in the optimisation of the

cement properties lies therefore in the determination of the proper gypsum dosage (see

chapter 4.3).

Blended cements

The most critical point of the blended cements in terms of cement performance is the

decrease in early strength. The dosage of mineral components in general purpose

applications, where a similar strength development as for the Portland cement has to beachieved, is therefore limited.


"Holderbank" Cement Seminar 2000Materials Technology 111 - Optimised Cement Design

!M1.»:1:MJPM

The possible dosages in such applications are the highest for the latent hydraulic mineral

components and go gradually down for the pozzolanic and inert materials. Typical

proportioning limits for the main mineral components in the cement are:

blast furnace slag: 30 - 40%

fly ash (class F): 15-30%

natural pozzolan: 1 5 - 30%

limestone. 10-20%

For blended cements used in special applications related to low heat evolution anddurability, the early strength development is not of primary importance and the dosages of

the mineral components can be higher. Some guide values on the proportioning in these

applications are given in Table 5. It has, however, to be mentioned that always specific tests

should be carried to verify the compliance with the application requirements.

Table 5: Guide values for proportioning of mineral components in cements for

special applications

Blast furnace Fly ash Natural pozzolan

slag (class F)

Low heat of hydration > 50% > 30% > 30%

Suphate resistance > 70% >30% > 30%Low chloride permeability* > 60% > 40% > 40%Avoidance alkali-aggregate > 40% > 25% >15%reaction

*provided the w/c-ration in concrete is sufficiently low

The actual proportioning of the mineral components in all applications will obviously also

depend on the selected cement fineness and on the permitted contents specified in the

respective standards.

4.3 Gypsum

(see also paper on cement hydration)

Portland cements

In Portland cements, the gypsum dosage has to be adjusted to the reactivity of the clinker

(i.e. C3A and alkali content) and the cement fineness to ensure proper set retardation.

Further adjustments may be necessary depending on the obtained grain size distribution

and the grinding temperature in the cement mill.

It is usually assumed that the gypsum dosage for proper set retardation is more or less

equivalent to the one required for best strength development and volume stability of the

cement. A practical method to find the optimum gypsum content is described in the ASTMstandard C 563 ("Standard test method for optimum S03 in Portland cement"). It is well

possible that the S03 content at the optimum gypsum content would even be above the

maximum value given by the standards.

In case that the Portland cement shows false setting tendency and problems with flowability

and storage stability, the gypsum content should be lowered or part of the gypsum should

be replaced by natural anhydrite. Replacement levels of up to 60% are possible for all type

of clinkers without having any problems with set retardation.


'Holderbank" Cement Seminar 2000—WW 1 ™;'*)^*


Blended cements

In case of blended cements, the situation gets much more complex and there exists no clear

procedure on how to adjust the gypsum dosage. Studies at HMC have indeed shown that

the optimum gypsum content has to be evaluated for each individual cement type.

Nevertheless, the findings indicated that the optimisation of the gypsum content in blended

cement can be a very effective means for the improvement of the cement quality.

Special attention in the determination of the optimum gypsum content has to be given to

cements, which contain limestone filler. In such cements, it may be possible to reduce the

gypsum content, since limestone acts also as a set retarder.

4.4 Chemical admixtures

Grinding aids are added at the cement mill at dosages, which are generally below 500 g/t.

The determination of the optimum dosage for a specific grinding aid depends mainly on the

cement fineness and the characteristics of the cement mill.

The dosage of the performance modifiers will essentially be determined by the objective for

their use. The main purpose of such admixtures is generally to achieve a desired cementperformance or to maintain the cement quality at a lower clinker content.

5. CEMENT GRINDING

5.1 General

The cement components have to be ground to fine particles, in order to attain the required

cementitious properties. The fineness after grinding is usually characterised by the specific

surface area or by the particle size distribution (PSD). The type of cement mill used canhave a considerable effect on the PSD.

During the grinding process with the traditional systems (ball mills), only a small portion of

the introduced energy is consumed for the comminution of the cement particles. A large

quantity of heat is set free and the temperature of ground cement increases appreciably. In

the modern grinding systems, less heat is produced, resulting in lower cement temperatureduring grinding.

Both, the fineness and the temperature of grinding are principal factors in determining the

cement properties.

5.2 Description of fineness

5.2.1 Specific surface area

The specific surface area of cement is usually determined by the Blaine method. The Blaine

value is calculated from the air permeability of a cement sample compacted under defined

conditions. The resistance to air flow of a bed of compacted cement depends on its specific

surface. The Blaine specific surface is not identical with the true specific surface of the

cement, but it gives a relative value which suffices for practical purposes.


"Holderbank" Cement Seminar 2000'


An absolute measurement of the specific surface can be obtained by the nitrogen (or water

vapour) absorption method - BET. In this method, the "internal" area is also accessible to

the nitrogen molecules and the measured value of the specific surface is therefore

considerably higher than that determined by the air permeability method:

Method Blaine Nitrogen Absorption (BET)

Cement A 2'600 cm2/g 7'900 cm2/g

Cement B 4'150cm2/g 10'000cm2/g

The Blaine value can sometimes be misleading, especially in the case of outdoor stored

clinker, blended cements - consisting of a more easily grindable component - and clinkers

containing underburnt material which is easier to grind. The properties of such cements can

often be poorer compared to other ground to the same specific surface.

5.2.2 Particle size distribution

Cements of the same specific surface may have different PSD and different properties.

Thus, the specific surface is not the only fineness criterion determining the properties of a

particular cement composition.

The determination of the PSD can be carried out by the following methods:

mechanical sieving (residues on sieves of a definite size (e.g. 32, 45 and 60 p.(i))

laser and sedigraph (residues over the whole range of particles sizes)

The mechanical sieving is usually applied in the cement plants. Due to the limitations in

sieve sizes, this method does not allow to measure the whole range of particle sizes.

The overall particle size distribution of cement is commonly analysed by means of the

theoretical distribution according to Rosin-Rammler-Sperling (RRS), which is described bethe following formula:

ln[ln(100/Rd)] = n[ln(d)-ln(d')]

being:

Rd = % of particles with diameter greater than d (residue)

d = particle size in urn

d' = characteristic diameter in urn (36.8% of the particles greater than d')

n = slope of RRS straight line

The data obtained in the particle size analysis is accordingly plotted in a so-called RRS-diagram (see Figure 7), having a double logarithmic ordinate (y-axis) and a logarithmic

abscissa (x-axis). After linear regression of the particle size distribution, the slope n of the

straight line and the characteristic diameter d' (at 36.8% residue) can be calculated.

The slope n and diameter d' are the significant values for the particle size distribution. Thefirst characterises the degree of distribution (wide-narrow), whereas the second one states

its location and is an indicator for the overall fineness. High n values results from a narrow

PSD and low d' values from a high overall fineness.

Differences in the PSD of cement can also be seen in the relation between the traditionally

measured Blaine values and sieve residues. At same sieve residue, the Blaine tends to be

lower with higher n values (see also Figure 8).



; r.n .1:1 :.]=?,vTraa

Figure 7: Particle size distribution of cement in RRS-diagram

IntlnflOO/R))4

Figure 8: Correlation between Blaine and residue 32 um for different n values

(data FLS)

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2000 2500 3000 3500 4000 4500

Fineness [cm2/g] Blaine

RRSB

n=H

0.8

0.9

1.0

1.1

1.2

5000


'Holderbank" Cement Seminar 2000'


5.3 Particle size distribution in the different grinding systems

The grinding in the modern cement mills goes together with a narrower PSD of the

produced cements. Due to the more efficient grinding process, less under- and over-size

particles are produced, which obviously leads to a shorter particle range and to higher

steepness of the PSD.

The steepness n as expressed by the RRS-distribution ranges from 0.8 for an open circuit

ball mill up to 1.2 for the newest grinding systems like vertical mill. Typical n values of

cements ground in various industrial mill systems are indicated in Table 6.

The characteristic diameter d' of commercial cements produced in the different grinding

systems varies typically between 10 and 30 urn. For identical specific surface, the d' values

are lower in the systems which give a narrower PSD, that means that the overall fineness of

the cement at same Blaine will be higher. At same d', the Blaine will be lower when the PSDgets narrower.

Table 6: Typical n values for PSD of cements ground in various industrial mill

systems

Mill type Steepness n of RRS-distribution

Ball mill (open circuit)

Ball mill (closed circuit)

Ball mill (high efficiency separator)

Vertical mill, roller press, Horomill

0.8 - 0.9

0.9-1.0

1.0-1.1

1.1-1.2

As mentioned before, the different PSD of the various grinding systems are also reflected in

the relationship between Blaine and sieve residues (e.g. on 45 urn), which are the usual

fineness measures applied in practice. The corresponding trends observed in the moderngrinding systems compared to the traditional ball mills are as follows:

lower Blaine at same sieve residue or

lower sieve residue at same Blaine

It is important to mention that, in view of certain quality problems experienced with a too

narrow PSD, the actual tendency for the new grinding systems is to adjust the PSD to asomewhat wider distribution.

5.4 Grinding of Portland cements

5.4.1 Influence of Blaine fineness

Since the hydration starts on the surface of the cement particles, it is the specific surface

area of Portland cement that largely determines the rate of hydration and thus the setting

and hardening rate. To achieve a faster hydration and strength development, rapid-

hardening cements are ground finer than ordinary Portland cement. It is common practice to

produce cement of various strength classes from one clinker by altering the fineness to

which it is ground. The Blaine value of cement varies between 2'500 cm2/g for ordinary

Portland cement (type I, ASTM) and 5'000 cm2/g for high early strength cement (type III,

ASTM).


'Holderbank" Cement Seminar 2000Materials Technology 111 - Optimised Cement Design

The rate of hydration is slowed down by the presence of cement gel and if a large quantity

of gel is formed rapidly, because of a large cement surface, the inhibiting action of the gel

soon takes place. For this reason, extra fine grinding is efficient only for the early strength

up to 7 days. Moreover, the rate at which the strength of concrete increases is substantially

lower than that of mortar (see Figure 9).

Considering the energy consumption for grinding, the fine grinding is often not economically

feasible. In those cases, where high early strength is not required, fine grinding is of little

value (see Figure 10). A large number of concrete applications are unable to exploit the

effects of fine grinding.

The relations between the Blaine fineness of cement and concrete properties can besummarised as follows:

1) Increasing the fineness of cement, reduces the amount of bleeding in concrete (see

Figure 11).

2) Increasing the fineness of cement above 3000, increases somewhat the water

requirement of concrete. Compared to the influences other than cement on the water

requirement of concrete, the influence of cement fineness is considerably smaller.

3) The strength of concrete is influenced by the fineness of cement. The early compressivestrength increases with an increase in cement fineness. The difference in compressivestrength due to the difference in fineness of cement, is considerably smaller at 28 daysand at later age (see Figure 9).

4) The fineness of cement influences the drying shrinkage of concrete. When the watercontent is increased because of fineness, the drying shrinkage is increased.



i!Mi.»:i:Mrraai

Figure 9: Effect of cement fineness on strength of mortar and concrete

CN

EE

05c«

CO

«]>en(A

aEoo

60

40 -

20

60

40

20

0Li

ISO- MORTAR

± ±3000 4000

CONCRETE

365 D90 D28 D

7D

2D

5000

900•-28D

1D

350 kg cement/m 3

Slump 5-7cm

_L J- ± ± 13000 4000 5000

Blaine, cm2/ g


"Holderbank" Cement Seminar 2000Materials Technology HI - Optimised Cement Design

!MMJ:i:M?IT

Figure 10: Relative specific energy consumption and compressive strength

development

SPECIFIC ENERGY'FOR GRINDING

MORTAR/strength

100h<

3000

CH

4000

USA

CONCRETESTRENGTH

MORTAR^STRENGTH

CONCRETESTRENGTH

AFTER1 DfflT

AFTER28 DAYS

5000 Blaine (cjn2/gr)



:t.».1=MtM?P

Figure 11: Effect of cement fineness on bleeding of concrete (non air-entrained

concrete, w/c-ratio = 0.57)

Fineness (Blaine)

5.4.2 Influence of particle size distribution

The influence of the fineness on the cement properties can be described more precisely,

when taking into account the PSD of the Portland cement. The PSD is of particular

importance with respect to workability and strength development.

The workability of Portland cement and concrete may impair when the PSD becomesnarrower (at constant specific surface). On one hand, the water requirement for a certain

consistency tends to increase and, on the other hand, the faster conversion of aluminate at

narrow PSD may lead to early stiffening problems.

The mentioned stiffening problems may especially occur if clinkers of high reactivity (high

C3A and alkali content) are ground together with the gypsum at low temperatures (little

formation of easily soluble sulphates), as it is the case in the modern grinding systems. With

such clinkers, the proper adjustment of the wideness of the PSD and/or of the calcium

sulphate carrier is therefore important.

The effect of the PSD of Portland cement on strength development is not always clear. Thegeneral trends can be summarised in the following way:

The most valuable particles for early strength are the ones between - 8 urn. The Blaine

value is thus a good indicator in this respect, as it is proportional to the portion in this

fraction.



iMt.u-A-.hvnaa

The 28 day strength is mainly controlled by the amount of particles in the range between2 - 24 urn, which is proportional to steepness n of the PSD.

The increase in the steepness n at a given Blaine is accordingly an effective means in

improving the strength potential at 28 days as illustrated in Figure 12. The positive effects of

higher n values are, however, less pronounced on concrete.

Figure 12: 2 day and 28 day compressive strength of Portland cement, as afunction of the specific surface area and the slope of the RRS-distribution of the cement

70

60

50

40

a.

Slope n:

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1

1000 2000 3000 4000

Specific surface of cement (cm2/ g)

6000




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5.5 Grinding of blended cements

5.5.1 General

The properties of blended cements are decisively influenced by the fineness of the cement

and its components respectively. Blended cements must generally be ground to a higher

overall fineness than Portland cements to maintain a similar strength development.

The grinding behaviour of the different components in blended cements may vary quite

significantly as illustrated in Figure 13. At constant fineness, the softer materials like

limestone and natural pozzolan yield a wider PSD than the clinker and blast furnace slag.

Despite its worse grindability, the grain size distribution of blast furnace slag does, however,

not differ too much from the one of clinker. The mentioned differences in grindability are of

great significance in the grinding of blended cements.

Figure 13: Steepness of the RRS-distribution of ground blast furnace slag, clinker,

pozzolan and limestone at same characteristic diameter d' in function of

the grindability index

3.O

n 0.8o

EECO

Of

Lco

O(E

oCO(0CDca.a>a

0.6 -

0.4

0.220 40 60 80

Grindability index in cm2/(g-s)

100 120

Blast furnace slag Clinker Pozzolan Limestone

• o *

The question to which fineness the components of blended cements shall be ground to

obtain optimum cement properties is often debated. The general concept of HMC is that the

hydraulic potential of the clinker should be used as much as possible by grinding it to a

sufficiently high fineness. The mineral components may be ground coarser, but latent

hydraulic and pozzolanic materials must still have a sufficient fineness to be suitably

activated to provide good final strength.




5.5.2 Compound grinding

The compound grinding of clinker, gypsum and mineral component(s) is still the most

common practice for the production of blended cements. The combined grinding with

mineral components softer than the clinker like limestone will widen the grain size

distribution of the resulting blended cement, whereas the mixture of clinker with a harder

material like blast furnace slag will give a somewhat steeper PSD than the ground clinker

alone. The different PSD can be explained by the fact that for compound grinding, the

harder material is enriched in the coarser fraction and the softer material in the finer

fractions of the cement.

In compound grinding, the different cement components can accordingly not be ground

individually or independently from each other. For a given grinding system, the fineness of

the components is pre-determined by their respective grindabilities; it is thus not possible to

adjust freely their fineness. The inevitable enrichments of certain components in the fine or

coarse fraction of the blended cement are, however, reduced with the modern grinding

technologies.

This lack of flexibility in compound grinding may limit the optimisation of the properties of

blended cements. With soft mineral components, the clinker will always remain rather

coarse, in particular at higher replacement levels, so that its hydraulic potential can not be

fully exploited. On the other hand, there might be an overgrinding of the mineral componentlike in the case of the natural pozzolans leading to an increase in water demand.

In case of slag cement, the clinker will get indeed finer and contribute as desired to the

strength development. The slag may, however, not be adequately refined and activated.

5.5.3 Separate grinding

Separate grinding of blended cements gives more flexibility in the design and optimisation of

the cement quality than compound grinding, since it permits free choice of the fineness of

the cement components. Nevertheless, the opinions on the real benefits of separate

grinding are still controversial.

In the following, the experience with separate grinding for the most relevant blended

cements containing blast furnace slag, fly ash, natural pozzolan and limestone is discussed.

5.5.3. 1 Slag cements

The studies on separate grinding of slag cement revealed that the fineness of the clinker

and slag influence the cement quality in the following way:

the clinker fineness is mainly related to early strength. In cements with low slag content

(<30%), the clinker fineness will also have an impact on final strength

the slag fineness determines mainly the final strength (at very high fineness, also

significant contribution to early strength possible). For good workability of the cements,

the PSD of the slag should not be too narrow.

In separate grinding, it is thus in principle possible to fine tune the strength curve according

to the above relationships. When the grinding energy is kept constant, the observed

improvements compared to intergrinding do, however, not seem to be too significant.

An interesting advantage for separate grinding may be in some cases the activation of the

slags through very fine grinding. Recent studies have shown that at Blaine finesses higher

than 4000, there is a considerable potential to improve the strength development at all ages(see also Figure 14).




'HOLDERBANK'

Figure 14: Influence of Blaine fineness of slag on compressive strength of ASTMmortar for cement with 40% slag

3500 4000 4500 5000

Blaine slag [cm2/g]

5500 6000

3 days 7 days 28 days



:Mi.»:ii?.-i?rCT

5.5.3.2 Fly ash cements

The separate grinding of fly ash cement as such has hardly been investigated. It appears

that at constant grinding energy separate grinding gives certain possibilities to fine tune the

final strength development of the fly ash cement.

Presently the most appropriate solution to produce fly ash cements is to add the fly ash to

the separator of the grinding system. The example in Figure 15 of a cement containing 16%fly ash demonstrates that in terms of quality and consumption of grinding energy, this seemsto be the best solution, also compared to intergrinding.

Figure 15: Influence of treatment undergone by the fly ash on the strength of

cement with 16% fly ash

60

50OPC30

40

£a

SE.30(0Q.

20

10

OPC40

28 d

addition to separator

intergrinding

mixingOPC40

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10 20 30

kWh/t cem40 50

5.5.3.3 Pozzolanic cements

Separate grinding of cements with natural pozzolan gives a somewhat higher early strength

and lower final strength than intergrinding at constant energy input. This relationship seemsquite logical as in intergrinding the clinker responsible for the early strength remains rather

coarse and the pozzolan contributing to the final strength is refined.

The studies on separate grinding of pozzolanic cements carried out at "Holderbank" showeda certain potential to increase the clinker factor at same cement quality, though at the

expense of a higher grinding energy. A typical increase might be in the order of 5% as it is

illustrated by the comparison of intergrinding and separate grinding for a pozzolanic cementfrom Mexico in Table 7.



Materials Technology 111 - Optimised Cement Design

!M|.H:i=M?ra

Table 7: Comparison of intergrinding and separate grinding for pozzoianic

cement from Mexico

Physical and mechanical properties Compound grinding (ball mill) Separate grinding

(ball mill and vertical mill)

Cement

Pozzoian (%)

Blaine (cm2/g)

n(-)

R 45 pm (%)

20

4170

1.0

3.3

25

4120

x)

7.5

Paste ASTM28.3

160

225

27.0

145

175

Water demand (%)

Setting time (min.)

- initial

- final

Mortar ASTM0.52

11.0

20.9

27.8

35.0

0.53

11.6

20.7

26.0

34.8

w/c-ratio

Compressive strength (MPa)

- at 1 day

- at 3 days

- at 7 days

- at 28 days

R 45 Mm = 20.9%R 45 Mm = 3.1%

Blaine = 2170 cm2/gBlaine = 4300 cm2/g

x) Pozzoian: n = 0.95

Clinker/gypsum: n = 0.95

5.5.3.4 Limestone cements

Studies on separately ground limestone showed that the limestone fineness as such has

virtually no influence on the strength development. The PSD of the limestone powder can,

however, play a role with regard to the workability characteristics of the limestone cement: a

wide distribution is in this respect more favourable than a narrow one.

In combined grinding with clinker, the limestone is automatically ground to the favourable

wide PSD. This quite advantageous behaviour in intergrinding, at least at lower limestone

dosages (up to 20%) may also explain the fact that separate grinding of limestone cement to

improve cement quality has usually not been applied in practice.



mu:i-'m.M:i:?M\!UM

5.6 Temperature and moisture conditions

The grinding of cement influences the properties of cement not only through an increase in

fineness, but also through the reactions taking place in the cement mill. Depending on the

temperature and moisture conditions prevailing in the mill, dehydration and hydration occur

which influence the grinding process, flowability, lump formation in silos, setting andhardening of cement.

Due to the heat liberated during the grinding process, the temperature in the traditional ball

mills rises to temperatures above 100°C. in the modern grinding systems, the grinding

temperatures are significantly lower (down to 50 - 60°C). In a particular mill, the exit

temperature of cement can vary in a wide range, in function of the inlet temperature of

clinker, cooling conditions and fineness of grinding.

The effect of the grinding temperature on the cement properties is mainly related to the

dehydration of gypsum. With increasing temperature, gypsum (CaS04 2H20) gets unstable

and transforms to hemihydrate and anyhdrite III under the release of water. The dehydration

of the gypsum will not only depend on the temperature, but also on the time the gypsum is

exposed to this temperature (see Figure 16). Another factor of less importance for gypsumdehydration is the humidity in the mill atmosphere (see Figure 17).

Figure 16: Influence of temperature on the dehydration of gypsum

150°C ^-140°C ^130°C

120°C

TIO°C

10 20 30Time of exposure(minutes)




:MM=N=MJrcM

Figure 17: influence of humidity on the gypsum dehydration.

Dry air

Dew point 20° CDew point 40° CDew point 70° CDew point 100

c

20 40Time of exposure (minutes)

60

According to the degree of dehydration, the gypsum will exert a different influence on the

cement properties (see also paper on cement hydration). If great part of the gypsum is

converted to the more easily soluble hemihydrate and anhydrite III, there may be some



!MI.H:l:MJITgB

problems with false setting in case a clinker of low reactivity is used. On the other hand, atoo low degree of dehydration may lead to flash setting tendency with reactive clinkers.

The dehydration of the gypsum may also have an impact on the storage stability of the

cement. If the cement enters the silo with a high temperature (80 - 90°C), further water canbe released form the still not dehydrated gypsum and lead to the hydration of the cement.These hydration reactions can cause lump formation and affect the strength of the cement(see Figures 18 and 19). Clinkers with high C3A and alkali content are particularly subject to

hydration reactions during storage.

Figure 18: Lump formation in a storage sensitive cement after one week storage at

various temperatures

+

4->

c

e

o

c•H

CO

A

10

8

6

2

J L 120 40 60 80 100 120 °CTemperature of storage



![.».»;J:MJT7aa

Figure 19: Compressive strength of a storage sensitive cement after one weekStorage at various temperatures

+J

cQ)

U

w

>•HOXN(0 £0) E

oo

w>1

•'0

i

50

40

30

20

10-

1 1 i 1 1 J ^20 40 60 80 100 120 °CTemperature of storage

Some measures to ensure the storage stability of the cement are:

low cement temperature in the silo

short storage time

reduction of gypsum content in the cement

substitution of gypsum by natural anhydrite

The storage stability is usually less problematic with the new grinding technologies.where

the temperatures of the cement coming from the miss is generaly low. The cement

temperatures in ball mills can be lowered by cooling the cement during the grinding process

(e.g. water injection) or by installing a cement cooler after the mill. For the cooling by meansof water injection, the temperature in the mill should always be kept above 100°C.

Otherwise, prehydration of the cement and strength losses may occur.


rTT5».»:l:M?ir'Holderbank" Cement Seminar 2000


6. LITERATURE

Cement components and cement properties

Gebauer, J., Kristmann, M., The influence of the composition of industrial clinker on cement

and concrete properties, World Cement Technology, March 1979, pp. 46 - 51

Wolter, H., Production, properties and applications of Portland slag cements and blast

furnace cements, Concrete Workshop, Queensland Cement Ltd., 1994

Malhotra, V.M., Ramezanianpour, A.A., Fly ash in concrete, Second edition.CANMET,

Ontario, 1994,307 pp.

Massazza, F., Pozzolana and pozzolanic cements, in: Lea's Chemistry of Cement and

Concrete, Fourth Edition, Arnold, 1998, pp. 471 - 631

Cochet, G, Sorrentino, F., Limestone filled cements: properties and uses, in: Progress in

Cement and Concrete, Volume 4, Mineral admixtures in cement and concrete, ABI, NewDelhi, 1993, pp. 266-295

Grinding and cement properties

Bapat, J.D., Higher qualities from modern finish grinding processes, International CementReview, January 1 998, pp. 54 - 56

Montani, S., Influence of grinding on the properties of blended cements, 34th Technical

Meeting, Davos, 1996, PT 96/14'096/E

Albeck, J., Kirchner, G., Influence of process technology on the production of market-

orientated cements, Cement-Lime-Gypsum, No. 10, 1993, pp. 615 - 626

Gebauer, J:, Cement grinding and quality problems, 32nd Technical Meeting, Montreux,

1992, VA92/5972/E

Schiller, B., Ellerbrock, H.-G., The grinding and the properties of cements with several main

constituents, Cement-Lime-Gypsum, No. 7, 1992, pp. 325 - 334

Ellerbrock, H.-G., Deckers, R., Mill temperature and cement properties, dto., No. 1, 1988,

pp. 1 - 12

Sprung, S., Kuhlmann, K., Ellerbrock, H.-G., Particle size distribution and properties of

cement, dto., Part I: No. 4, 1985, pp. 169 - 178, Part II: No. 9, 1985, pp. 528 - 534

Sprung, S., Influence of process technology on cement properties, dto., No. 10, 1985, pp.

577 - 585


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